Quadrupole mass spectrometer having resolution variation capability

ABSTRACT

A quadrupole mass spectrometer includes an ionization chamber, an ion detector, and a quadrupole structure having four electrodes extending therebetween. A d.c. voltage U and a high frequency voltage of amplitude V are linearly superimposed and are applied to the electrodes while maintaining the relationship U Alpha MV - K, where Alpha M denotes a critical U/V ratio. This constrains the mass peaks of a spectrum to be of substantially uniform shape. Variation of the parameter K permits the spectrometer resolution to be varied. In accordance with one aspect of the present invention, the d.c. voltage is maintained constant at values lower than the peak value of the stability curve for atomic hydrogen in a range of smaller values of the voltage amplitude V. The spectrum for atomic hydrogen can thereby be isolated.

United States Patent 1191 Sakai et al.

[ Jan. 8, 1974 QUADRUPOLE MASS SPECTROMETER 3,413,463 11/1968 Brubaker250/419 HAVING RESOLUTION VARIATION CAPABILITY Primary Examiner-WilliamF. Lindq uist [75] Inventors: Tadashi Sakai; Masahiko Sakirnura;Attorney-Samoa Hopgood and Cahmafde Yoichi Ino, all of Tokyo, Japan [73]Assignee: Nippon Electric Varian, Ltd., [57] ABSTRACT Tokyo, Japan Aquadrupole mass spectrometer includes an ionization chamber, an iondetector, and a quadrupole [22] Flled' 1971 structure having fourelectrodes extending therebe- [21] Appl. No.: 123,687 tween. A d.c.voltage U and a high frequency voltage of amplitude V arelinearlysuperimposed and are ap- L plied to the electrodes while maintaining therelation- [30] Forelgn Apphcauon Pnorlty Data ship U=a V K, where a Mdenotes a critical U/V Mar. 14, I970 Japan 45121704 ratio Thisconstrains the mass peaks of a Spectrum to July 18, 1970 Japan 45/634l0be of substantially uniform shape variation of the rameter K permits thespectrometer resolution to be [52] US. Cl 250/413 DS varie [51 Int. Cl.I-I0lj 39/34 In accordance with one aspect of the present [58] F leld ofSearch 250/41 9 DS invention, the d.c. voltage is maintained constant at1 1 'liliiiliifiilifii 212122?ZLZBTJSZILQZZFIZZ UNITED STATES PATENTSvoltage amplitude V. The spectrum for atomic 2,950,389 8/1960 Paul el al.L 250/413 hydrogen can thereby be iso]a[ed 3,280,325 l0/l966 Brunnee etal. 250/419 3,321,623 5/1967 Brubaker et al. 250/419 5 Claims, 18Drawing Figures l3 |4 Amplltude Slgnol Control Amplifier Generator 1Get.

l2 Tooth VOliOgE 5 Generator Generator Ion Curreni- PATENIEDm 8 I9743.784.814

sum 10? 5 lNl/E/VTORS TADASHI SAKAI MASAHIKO SAKIMURA YOICHI mo ATTORNEYPATENTED JAN 8 i974 SHEET MJF 5 TADASHI MASAHIKO SAKIMURA YOlCHl 1N0ATTOR EYS 1 QUADRUPOLE MASS SPECTROMETER HAVING RESOLUTION VARIATIONCAPABILITY DISCLOSURE OF THE INVENTION This invention relates to asystem for rendering the resolution of a quadrupole mass spectrometervariable and, more specifically, to a spectrometer system wherein thesensitivity ratio for differing masses in the mass spectrum alwaysremains unchanged, even when the resolution is varied.

A quadrupole mass spectrometer, used for determination of thecomposition of chemical substances, comprises an ionization embodimentsa quadrupole electrode structure, and an ion detector. A chemicalsubstance to be analyzed is introduced into the ionization chamber inits gaseous state, and is subjected to an electron bombardment forionization. The resultant ions are directed to the quadrupole structure.In the quadrupole structure, only those of the ions which have aspecific range of charge vs. mass ratio are permitted to passtherethrough. Those ions are collected by the ion detector forconversion into an electric current of a magnitude corresponding to thenumber of ions which have passed through the quadrupole structure.

The charge to mass ratio of the ions collected by the ion detector isdetermined by the amplitude and frequency of a high frequency voltageapplied to the electrodes of the quadrupole structure. Hence, bychanging the magnitude or frequency of the applied voltage, a displaycan be obtained on an oscillograph orrecorder of the number of the ionsdetected as a function of mass.

In order to determine the composition of the substance from the massspectrum thus displayed, it is necessary to compare such a spectrumagainst a reference spectrum obtained for a substance of known mass.During the comparison, it is desirable to have mass spectral peaks of anidentical shape on the spectrum.

It has been the practice in providing a mass spectrum display to varythe voltage or frequency thereof over the whole mass range whilemaintaining constant the ratio of a d.c. voltage U to the amplitude V ofthe high frequency voltage applied to the quadrupole electrodes. Thisratio is generally identified by the symbol a (alpha). This results inaspectrum having spectral peaks which have an increasing peak width (Am)with an increase of mass (m), with the result that peaks of an identicalshape cannot be obtained. Also, variation of the resolution results in asubstantial change in the ion sensitivity ratio.

An improvement has been proposed which comprises structure for adjustingthe voltage ratio a for matter of differing mass, as will be describedlater. This proposal involves practical difficulties in adjusting thevoltage ratio a to a proper value. A change in the a-value gives rise toa change in the peak width Am, for the same mass, making the compositesystem quantitatively unreliable.

It is therefore an object of the invention to provide a variableresolution system for a quadrupole mass spectrometer which allows a massspectrum to be readily obtained with the peak width Am being completelyindependent of the mass m.

It is another object of the invention to provide a variable resolutionsystem for a quadrupole mass spectrometer wherein resolution may bevaried while maintaining the ion sensitivity ratio unchanged for ionshaving different masses.

It is a further object of the invention to provide a variable resolutionsystem for a quadrupole mass spectrometer which effects a cleardistinction of the mass spectrum for hydrogen atoms vis-a-vis massspectra for other substances.

In accordance with the above and other objects of the present invention,a quadrupole mass spectrometer comprises an ionization chamber, an iondetector and a quadrupole structure having four electrodes extendingtherebetween, wherein a d.c. voltage of magnitude U and the amplitude Vof a high frequency voltage, both applied to the electrodes, are variedin a manner to satisfy the relationship U aM V K. The spectrometerresolution is varied by changing the value of the parameter K. In thismanner, the respective mass peaks on the spectrum are given asubstantially identical shape.

For small values of V, or under the conditions 0 V Kla H, theabove-mentioned relationship is modified to U H, and thus madeindependent of V. The value of the parameter H is fixed at a valuesmaller than the peak value of the stability curve for a hydrogen atom.This avoids the intersection of the stability curve for hydrogenmolecules with the line represented by the relationship U H at smallvalues of V. As a consequence, in the stability region of hydrogen atomsand for lesser values of V before reaching such a region, the stabilityregion for hydrogen molecules lies below the boundary defined by therelationships U H and U a V K, so that the spectrum for hydrogen atomscan be obtained in an isolated form.

The above-mentioned and other objects, features and advantages of theinvention will become apparent from the following detailed descriptionof the present system taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 schematically shows a perspective view of a quadrupole structureof a quadrupole mass spectrometer to which the invention is applicable;

FIG. 2 shows a group of curves illustrating the condi tions required topermit certain ions to pass through the quadrupole structure shown inFIG. 1;

FIG. 3 shows an example of a mass spectrum;

FIG. 4 shows a group of curves for describing a typical prior art systemfor varying spectrometer resolution;

FIG. 5 shows a group of similar curves illustrating the operation of thevariable resolution system according to the invention;

FIG. 6 shows in block form a typical power supply unit which isapplicable to the spectrometer shown in FIG. 1;

FIGS. 7 and 8 are circuit diagrams, partially in block diagram form, offirst and second embodimetns incorporating the variable resolutionsystem of the invention illustrated in FIG. 5 and applied to the powersupply unit shown in FIG. 6;

FIG. 9 shows a group of similar curves for describing the operation ofthe improved variable resolution system in its lower mass numberportion;

FIG. 10 shows another group of curves illustrating the variableresolution system of the invention;

FIG. 11 is a circuit diagram of a third embodiment incorporating thepower supply to which the invention is applied;

FIGS. 12A through 12E are schematic diagrams illustrating the operationof the circuit shown in FIG. 11;

FIG. 13 is a block diagram of another typical power supply unitapplicable to the spectrometer shown in FIG. 1; and

FIG. 14 is a schematic circuit diagram of a fourth embodimentincorporating the variable resolution system illustrated in FIG. asapplied to the power supply unit shown in FIG. 13.

Referring now to the drawings and, in particular to FIG. 1, thequadrupole structure for a quadrupole mass spectrometer comprises foursolid cylindrical electrodes l, 2, 3 and 4 disposed in parallelrelationship with equi-angular intervals about, and spaced by a distancer from a center axis Z. Electrodes l and 2 are disposed on oppositesides with respect to the axis Z. Similarly, the other pair ofelectrodes 3 and 4 are disposed opposite to each other with the axis Zlying therebetween. Electrodes I and 2, and 3 and 4 are respectivelyconnected in common and supplied with voltages U V cos w t and U V cos(0 t, respectively, where the character U denotes a d.c. voltage, andthe character V, the amplitude ofa high frequency sinusoidal voltage ofangular frequency w, when w 21rf. The character t denotes time.

When an ion having a mass m and an electric charge e is generatedincident along the center axis Z of the structure, it comes under theinfluence of the electric field formed in the quadrupole structure, andthe components of force exerted on the ion can be represented asfollows:

e (U V cosw t) (2yl'yo By using the following denotations:

; ='w [/2, a 8eU/mro 0 q 4eV/mro (0 the above equations (1) and (2) canbe converted as follows:

d xldf (a 2q cos 25) x 0 d y/df (a 2q cos 26) y 0 apex located at thecoordinates (a 0.237, q 0.706). This region represents the ranges ofvalues of the parameters a and q for which an ion having a specificcharge vs. mass ratio e/m can repeat its stable oscillation. Theexterior of the approximate triangle represents the astable solutionregion, i.e., the ranges of values of the parameters a and q for whichthe motion of an ion is unstable with its amplitude rapidly increasingto result in a collision with one of the electrodes. In other words,ions that remain within the stable solution region can repeat stableoscillations and pass through the quadrupole structure, while all otherions are captured by the electrodes failing to pass therethrough.

For the purpose of further description, the ratio of the dc. voltage tothe amplitude V of the high frequency voltage will be hereafter denotedby a. Thus, a U/V=a/2q. This corresponds to a straight line in the a qplane, passing through the origin and having an inclination of 2a.Denoting the inclination of the line which passes through the apex ofthe approximate triangle shown in FIG. 2 by 20: we have 2a,, 0.3356,such a being referred to as critical ratio. When the inclination of 2ais less than 201M, the straight line will intersect with the approximatetriangle (stability curve) at points, the q-coordinates of which aredesignated by q, and q respectively. It will be seen from FIG. 2 that asthe value of a is varied, the difference between 1 and q and hence thedifference Am between m and m varies and becomes infinitesimal as aapproaches aM. When a is made close to 01M, the equivalent molecularweight M/n of an ion which can pass through the quadrupole structure isexpressed in terms of practical units as follows:

M/n V/7.2l9 fr where M denotes the molecular weight; n, charge number ofthe ion; V, the amplitude of the high frequency voltage in volts,f, thefrequency in MHz and r the distance mentioned above in centimeters.

Usually, the frequency f is maintained constant while the amplitude Vthereof is varied to allow passage through the quadrupole structure ofions having successively changing masses, thereby producing on a displayunit a depiction of an ion current as a function of mass. An example ofa mass spectrum thus obtained is shown in FIG. 3. The abscissa in thespectrum diagram indicates mass (or the amplitude V), and the ordinateindicates ion current.

In general, the resolution of a mass spectrometer is represented bym/Am, where m denotes the mass number at the center of a mass peak andAm, the width of the peak as shown in FIG. 3. The resolution m/Am of aquadrupole mass spectrometer for the voltage ratio a close to thecritical ratio aM is expressed as follows:

m/Am 0.75/1 (tr/01M) (7) as discussed by W. Paul et al. in the articlespublished in Zeitschriftftlr Physik, vol. 140, pp. 262-273 19 5) Theconventional method varies the high frequency i voltage V whilemaintaining the voltage ratio a uneaks obtained do not meanmama]"asteroids;

FIG. 4, there is shown the stability diagram depicted in the a-q planein FIG. 2 as transformed on the U-V plane. As shown, the stable regionsfor ions having masses ml, m2 and m3, permitting these ions to passthrough the quadrupole structure are represented by the approximatetriangles 7, 8 and 9, respectively, which are similar with respect toeach other, and which have the V-axis or abscissa as their base. Bydrawing straight lines U oz/V and U (1 V in FIG. 4, it will be seen thatthe segments Am intercepted by intersection of these straight lines withthe approximate triangles will increase with an increase in the mass mand also vary with the variation of the value of u even for the samemass.

It has heretofore been proposed in the prior art to vary the voltageratio a to produce an equal range of Am for all mass. However, thistechnique is impracticable because of the practical difficultiesinvolved in the adjustment of the voltage ratio a to a proper value. Itis extremely difficult, if not impossible, when the spectrum of asubstance having an unknown mass is to be compared with the spectrum ofa substance having a known mass, to establish an equal value of a forboth of these spectra. If the values of a differ from each other, thevalues of Am will be different even for the same mass number withresulting difference in the detected ion currents. This fact isprimarily responsible for the general belief that a quadrupole massspectrometer is not effective for quantitative determinations.

The variable resolution system according to the pres-' ent inventionwill now be described with reference to FIG. 5 in which the samestability curves 7, 8 and 9 for masses ml, m2 and m3 are shown as forthe case of FIG. 4, with their apices being joined by a straight linehaving an inclination at U/V. Another straight line is drawn in parallelwith the at, line and intersects the U-axis at K, below the origin (thisbeing designated as straight line I). A further straight line (straightline II) is drawn parallel to the or line and downwardly shifted by K2therefrom. It will be seen from the drawing that the length along theV-axis of those segments of the straight line I or II which areintercepted by the stability curves 7, 8 and 9, viz., the values of Aml,Am2 and Am3 as obtained from the straight line II, for example, becomeequal to one another as long as the straight line I or II intersects thestability curves adjacent to their peak points. In other words, thevalue of Am becomes constant, independent of ion mass, when U and V arevaried in a manner satisfying the relationship represented by thestraight line I or II, and its width is determined by the K-value alone;that is, the value of a d.c. voltage opposite in polarity to, and addedto the d.c. voltage U.

A mathematical analysis of the foregoing will be given below. Denotingresolution in terms of U and V by substituting for a in the form a U/ Vin the equam/Am= 0.75/(1l/aM,U/V)

The straight line I or II shown in FIG. 5 can be expressed as follows;

Substitution of the equation (9) into the equation (8) and solving theresult for Am leads to the following expression:'

Representing Am in terms of practical units by using the equation (6).

where AM denotes Am when expressed in terms of practical units. As willbe apparent from equation l l when the frequency f and the interelectrode spacing r0 are both fixed, AM depends on K-value alone, makingitself completely independent of the equivalent molecwhere B is aparameter, and in which both the parameters B and K are independentlyvariable. The relation presented in the equation (9) is obtained byestablishing the B-value substantially at the critical ratio aM.

Before proceeding with the description of such circuit, a typicalcontrol power supply circuit for driving a quadrupole mass spectrometerwill be described with reference to FIG. 6. One typical example of suchcontrol circuit comprises a signal generator 1 l for generating a highfrequency voltage of frequency f; a generator 12 for producing asaw-tooth wave voltage; an amplitude conrol circuit 13 operable tocontrol the amplitude of the high frequency voltage in accordance withthe saw-tooth voltage; a high frequency amplifier 14 for amplifying theoutput of the amplitude control 13 and producing voltages given by V cos2 1r ft and V cos 2 ft a d.c. voltage generator 15 for deriving d.c.voltages U and -U responsive to the output from the amplifier l4; andcircuit connection apparatus for linearly superimposing (adding) thed.c. voltages on the high frequency voltages and applying thesuperimposed voltages to the electrodes 1, 2 and 3, 4 of the quadrupolestructure, respectively.

A first embodiment of the circuit for constructing the variableresolution system of the invention comprises an improvement of the d.c.voltage generator 15 of the circuit shown in FIG. 6. Thus, asillustrated in FIG. 7 by way of example, the first embodiment thereforcomprises: an ac. voltage rectifier 17 for providing rectification ofVcos 271' ft and -V cos 21r ft to produce a d.c. voltage -rV proportionalto the amplitude V, where r denotes a constant; a variable resistor(potentiometer) 7 18 for providing voltage division of the d.c. voltage;a power source 19 for producing a variable d.c. voltage which has apolarity opposite to the aforesaid d.c. voltage and which biases thevariable resistor 18; and a pair of differential d.c. amplifiers 20 and21 for producing d.c. voltages U and U having the samemagnitudecontrolled by the divided d.c. voltage and having oppositepolarities. Variation of the movable point on the resistor 18 in thiscircuit results in a change in the value of B in the equation (12) andvariation of the variable d.c. power source 19 results in a change inthe value of K.

The variable d.c. voltage source 19 effecting a change of K-value may beconnected in a different position. Thus, as shown in FIG. 8, oneterminal of the variable resistor 18 may be grounded directly, with oneof the inputs of the respective differential d.c. amplifiers 20 and 21being connected with a variable d.c. voltage source 22 rather than beingconnected to the ground. The voltage sources 22 for the both amplifiers20 and 21 are linked together for interlocked variation. It will beevident that the circuit of FIG. 8 effects a similar function as thedircuit of FIG. 7.

It should be understood that the relation of the equation (12) remainseffective when the output from the rectifier 17 is taken out withpositive polarity by sup-- plying the output from the variable d.c.voltage source 19 or 22 having a negative polarity.

In this manner, the invention maintains the relationship U a V K as thevoltage V is varied to allow successive passage through the quadrupolestructure of ions having different masses for the purpose. of massspectroscopy. However, it will be noted that for smaller values of thevoltage V, U 0, so that in such an area, all of the ion mass will passthrough the quadrupole structure, thereby rendering it impossible toobtain isolated spectra of lower mass numbers. Thus, the stable regionfor atomic hydrogen having the lowest mass number, or m,,=l, exists at avery small value of the voltage V, as indicated by an approximatetriangle 30 in F l6. 9. The straight line ll starts from inside thestable region 30 for atomic hydrogen and passes thereacross with anincrease of the voltage V without thereafter again crossing thestability curve 30. Thus, for V which satisfies O V K /aM, U 0, all ofthe ions having the masses m m1, m2 and m3 will pass through thequadrupole structure. Thereafter, an increase in the voltage V producesastable regions for larger masses to successively inhibit passage ofcorresponding ions, subsequently followed by passage of atomic hydrogenions. However, since a further increase of the voltage V does notproduce another intersection with the stability curve for atomichydrogen to thereby delineate the region of its passage, it isimpossible to take out only atomic hydrogen ions in the form of its massspectrum.

Considering now the straight line I, all of the ions in the rangedefined by the inequality: O V K /aM pass through the electrodes 1-4.With an increase in the voltage V beyond this range, all of the ionsonce experience an astable region, and then there appears stable region30 for atomic hydrogen. At this time, the straight line 1 provides thestable region for only atomic hydrogen, and therefore atomic hydrogenions can be taken out in the form of a mass spectrum.

While the detection sensitivity for the ion current increases with themagnitude of Am, a choice of a value for Am which is equal to or greaterthan unity results in a partial overlapping of the mass spectra for themasses m and m+l Thus for the practical use of a mass spectrometer, thevalue of Am must not be greater than unity. It has been confirmed byexperiments that when K is varied to produce a value of Am in excess of0.5, there resulted a tendency toward the situation exhib ited by thestraight line ll, described above in connection with FIG. 9, i.e., themass spectrum for atomic hydrogen ions overlaps on its lower mass sidewith the mass spectra of other ions.

In order to obtain an isolated spectrum for atomic hydrogen with arelatively large value of Am and without upsetting the desirableidentity of Am for differing mass, a technique depicted in FIG. 10 maybe used to establish a relation between U and V. Thus, for a small valueof V within the range defined by the inequality O V K /aMH, a straightline lll defined by U H is used, while the straight line ll or U= a V- Kis used for a value of V greater than K /aM-H. The value of theparameter H is fixed at a value lower than the peak value U of thestability curve 30 for atomic hydrogen and so as to avoid theintersection in the range O V K /aM-H between the line U H and thestability curve 31 for hydrogen molecules H having a mass number thatexceeds that of atomic hydrogen by one. It will be noted that KJaM-Hrepresents the value of V at the intersection of the straight lines lland lll.

With this arrangement, for small values of V before the intersection ofthe straight line I]! with the stability curve 30 for atomic hydrogen,all of the ions m m,,,, ml, m2 and m3 are in their astable region andhence cannot passthrough the quadrupole structure. With an increase ofthe V-value into the stability region 30 for atomic hydrogen, onlyatomic hydrogen ions are allowed to pass through the quadrupolestructure. The straight line III is continued by the straight line llbefore entering the stable region 31 for the next higher mass or m l,i.e., molecular hydrogen. After going through the stable region 30.foratomic hydrogen, there once appears an astable region for all of theions before entering the stable region 31 for hydrogen molecules, sothat the mass spectrum for atomic hydrogen can be detected in anisolated form. The adjustment of H- value allows the peak width Am ofatomic hydrogen to assume the same value as the peak width of otherions, Am 1, Am2 or Am3. Variation of K-value with H- value fixed resultsin a variation of Am in the similar manner to the peak widths of themasses ml, m2 and m3, thus making it possible to vary the width Am ofthe mass spectra while maintaining Am at the same value as the peakwidth Am of other ions.

The variation of the voltages U and V in the manner depicted by thestraight lines ll and III in FIG. 10 may be achieved by a circuit shownin FIG. 11, which comprises a modification of the circuit shown in H6. 7wherein the voltage divider terminal 33 of the variable resistor 18 isconnected to one of the inputs of a high gain differential d.c.amplifier 35 through a resistor 34, rather than connecting the terminal33 directly to the aforesaid differential amplifiers 20 and 21. The d.c.amplifier 35 has its other input connected to the ground, and the output36 thereof is connected to the inverting input through a rectifier 37and a resistor 38. The resistors 34 and 38 may have an equal resistance.The inverting input and output 36 of the amplifier 35 are interconnectedby a rectifier 39 which has approximately the same characteristics asthe rectifier 37.

9 The output terminal 36 of rhpl'ir'iei'as'i'efinid with the anode ofthe rectifier 37 and the cathode of the rectifier 39. The junction 40between the rectifier 37 and the resistor 38 is connected to one of theinputs of the respective differential amplifiers and 21. Thesedifferential amplifiers 20 and 21 have their other inputs connected withvariable d.c. voltage sources 41 and 42, respectively, for supply of theabovementioned parameter l-I therefrom. Variation of the outputs fromthese d.c. sources 41 and 42 can be effected by interlocked (ganged)operation.

FIG. 12A shows the relation of the output, rV, from the rectifier 17with respect to the voltage V, the output being a straight line passingthrough theorigin O. This output, rV, is superimposed on the positivevoltage supplied from the d.c. source 19 to yield at the dividerterminal 33 a voltage, which is shown by a straight line crossing theV-axis at (V= K/aM, U O), as shown in FIG. 128.

For a value of V less than K/aM, or for the positive range of U, thedifferential d.c. amplifier 35 produces a negative output at theterminal 36, thereby bringing the rectifier 37 into a non-conductivestate while the rectifier 39 is rendered conductive, thereby maintainingthe terminal 40 substantially at ground potential. When V exceeds K/aMand U becomes negative, the amplifier 35 provides a positiveoutput torender the rectifier 37 conductive while the rectifier 39 becomesnon-conductive. Accordingly, there is produced at the output terminal 40an output voltage which corresponds in magnitude (but of oppositepolarity) to the voltage at the divider terminal 33. Thus, the inputvoltage U to the amplifiers 20 and 21, appearing at the ter-, minal 40,will have a relationship with respect to the voltage V as represented bya straight line, shown in FIG. 12C, which begins to rise from the pointgiven by V a The voltage U at the terminal 40 forms one of the inputs tothe differential amplifiers 20 and 21, the other input of which issupplied with an output --H from d.c. sources 41 and 42, respectively.Hence, for smaller values of V, outputs H and I-I are obtained, and fora value of V in excess of K/a -H, positive and negative outputscorresponding to the input U are obtained, the resultant outputs beingshown in FIGS. 12D and 12E. These are the requisite d.c. outputs U andU.

' In operation, K is set to zero by reducing the output voltages fromthe d.c. sources 19, 41 and 42 to zero. Under this condition, the highfrequency voltage V is swept at high speed while watching the massspectra, with the movable point on the variable resistor 18 varied fromsweep to sweep to thereby change the relationship U BV. In this manner,a point is determined for the position of the movable point on thevariable resistor 18 at which all of the mass spectra just disappear orappear, whereupon the adjustment of the variable resistor 18 is fixed,yielding B a 0.1678). Then the d.c. source 19 is adjusted to vary theK-value to establish the peak width Am. Subsequently, the d.c.

sources 41 and 42 are adjusted to vary the H-value and fix it at a levelsuch that the mass spectrum for atomic hydrogen can be obtained ianisolated form, desirably with its peak width Am being chosen equal tothe peak widt Am a so on 9 h were??? SW95};

In the foregoing description, the d.c. voltage U has been derived byrectifying the high frequency voltage. I It marks l k wisgqbtaine its"!the s yiemh 5 2192.

generator 12 shown in FIG. 6. One such example isshown in FIG. 13, inwhich there is provided a d.c. voltage generator 23 which is driven bythe output from the sawtooth voltage generator 12 to produce d.c.voltages U and U for combining with the high frequency voltages. Asexplained above regarding the application of the variable resolutionsystem described in connection with FIG. 5, the output of the sawtoothvoltage generator 12 may be directly supplied to the variable resistor18, eliminating the rectifier 17 from the circuit shown in FIG. 7.Similarly, the output from the saw-tooth voltage generator 12 may bedirectly coupled to the variable resistor 18 shown in the circuit ofFIG. 8 or FIG. 11.

Although in the above description the inclination a has been establishedfirst, followed by adjustment of the voltage K and the voltage I-Isuccessively to obtain the U-V voltage relationship illustrated in FIGS.12D and 12E, the circuits which establish these three parameters may bevaried in any desired order.

Thus it will be understood that according to the variable resolutionsystem of the invention, a desired value of peak width Am can beestablished for the masses by selection of a suitable magnitude for thed.c. voltage K, and this value of Am remains constant for all of thediffering subject masses. Stated differently, all the mass peaks havethe same shape and the ion sensitivity ratio voltage l-I. Further, thereis the capability of maintaining the peak width Am for atomic hydrogenthe same as the peak width for matter of other mass. Also for atomichydrogen, variation of the d.c. voltage K results in a change for thewidth of the mass spectrum for atomic hydrogen in the same way as thewidth of other mass spectra, whereby it is possible to achieve the sameconstant sensitivity ratio for atomic hydrogen having a mass number ofunity as for other mass numbers when apparatus resolution is varied.Therefore, the variable resolution system according to the inventiongreatly enhances the quantitativedetermination capability of aquardupole mass spectrometer, compared with prior art apparatus.

While several embodiments of the invention have been described above,various modifications thereof will be readily apparent to those skilledin the art without departing'from the spirit and scope of the presentinvention.

What is claimed is:

l. A variable resolution system for a quadrupole mass spectrometercomprising an ionization chamber, an ion detector and a quadrupolestructure having four electrodes extending between said detector andchamber; said electrodes being symmetrically disposed with respect tothe axis thereof; means for electrically connecting alternate ones ofsaid electrodes together to form two electrode pairs; said electrodepairs having direct current potentials of an opposite polarity appliedjthereto; wherein the improvement comprises: a variable d.c. source-forsupplying a d.c. voltage of a magnitude U to said electrodes of saidquadrupole structure,

a high frequency sinusoidal voltage source for supply- .-irs .hishfi wueltasaqfxaualeamp v to said electrodes, and means for varying the d.c.voltage U and the amplitude V of the high frequency voltage in a mannersatisfying the relationship U a V K, where a denotes a critical U/ Vratio at the limit of stable spectrometer response, and K denotes aconstant determining the peak width Am of a mass spectrum determined bysaid spectrometer; whereby all the mass peaks of the spectrum arecharacterized by a substantially identical shape.

2. A variable resolution system for a quadrupole mass spectrometercomprising an ionization chamber, an ion detector and a quadrupolestructure having four electrodes extending between said detector andchamber; said electrodes being symmetrically disposed with respect tothe axis thereof; means for electrically connecting alternate ones ofsaid electrodes together to form two electrode pairs; said electrodepairs having direct current potentials of an opposite polarity appliedthereto; wherein the improvement comprises: a source of high frequencysinusoidal voltage of variable amplitude V; means for producing a d.c.voltage proportional to the amplitude V of the high frequency voltage;variable resistor means forming a voltage divider for said d.c. voltageand for providing a second d.c. voltage of a value a V, where denotes acritical U/V ratio at the limit of stable spectrometer response; meansfor biasing said second d.c. voltage of magnitude a V with a third d.c.voltage of magnitude K, where K denotes a constant which determines thepeak width Am of a mass spectrum; and means for applying the highfrequency voltage and the resultant d.c. voltage of magnitude a VK tosaid electrodes of said quadrupole structure.

3. A variable resolution system for a quadrupole mass spectrometercomprising an ionization chamber, an ion detector and a quadrupolestructure having four elec- Q P sxtinivabst e n is detector n chamber;

said electrodes being symmetricallydisposed with respect to the axisthereof; means for electrically connecting alternate ones of saidelectrodes together to form two electrode pairs; said electrode pairshaving direct current potentials of an opposite polarity appliedthereto; wherein the improvement comprises: a source of high frequencysinusoidal voltage of variable amplitude V for application to saidelectrodes of said quad rupole structure; means for generating a d.clvoltage U for connection to said electrodes which remains at a constantvalue of H for values of V, wherein O V K/arH and which varies inaccordance with the equation U oqV-K for a value of V in excess ofK/a,,H, where 0: denotes a critical U/ V ratio at the limit of stablespectrometer response and K denotes a constant determining the peakwidth Am of a mass spectrum determined by said spectrometer; means forvarying the parameter K to vary peak width; and means for varying theparameter H to establish said parameter H at a value smaller than thepeak value of the stable spectral response for atomic hydrogen.

4. A variable resolution system for a quadrupole mass spectrometercomprising an ionization chamber, an ion detector and a quadrupolestructure having four electrodes extending between said detector andchamber; said electrodes being symmetrically disposed with respect tothe axis thereof; means for electrically connecting alternate ones ofsaid electrodes together to form two electrode pairs; said electrodepairs having direct current potentials of an opposite polarity appliedthereto; wherein the improvement comprises: a source of high frequencysinusoidal voltage of the variable amplitude V; means for generating ad.c. voltage proportional to the amplitude V of said high frequencyvoltage; voltage divider means for deriving, by voltage division of saidd.c. voltage, a second d.c. voltage of a value a V, where 0: denotes acritical U/V ratio at the limit of stable spectrometer response, meansfor biasing said second d.c. voltage with a third d.c. voltage of avalue K, where K denotes a parameter that determines the peak width Amof a spectrum; means for rendering said biased d.c. voltage a V K equalto a null output for a value of V between zero and K/a means forcomparing the output from said rendering means with a constant voltage Hand choosing the greater of the two as its output U, the value of Hbeing chosen less than the peak value of the stable spectral responsefor atomic hydrogen and means for applying said output U and said highfrequency voltage to the electrodes of the quadrupole structure.

5. A variable resolution system according to claim 4, further includingmeans for varying the magnitude of the voltage l-l.

1. A variable resolution system for a quadrupole mass spectrometer comprising an ionization chamber, an ion detector and a quadrupole structure having four electrodes extending between said detector and chamber; said electrodes being symmetrically disposed with respect to the axis thereof; means for electrically connecting alternate ones of said electrodes together to form two electrode pairs; said electrode pairs having direct current potentials of an opposite polarity applied thereto; wherein the improvement comprises: a variable d.c. source for supplying a d.c. voltage of a magnitude U to said electrodes of said quadrupole structure, a high frequency sinusoidal voltage source for supplying a high frequency voltage of variable amplitude V to said electrodes, and means for varying the d.c. voltage U and the amplitude V of the high frequency voltage in a manner satisfying the relationship U Alpha MV K, where Alpha M denotes a critical U/V ratio at the limit of stable spectrometer response, and K denotes a constant determining the peak width Delta m of a mass spectrum determined by said spectrometer; whereby all the mass peaks of the spectrum are characterized by a substantially identical shape.
 2. A variable resolution system for a quadrupole mass spectrometer comprising an ionization chamber, an ion detector and a quadrupole structure having four electrodes extending between said detector and chamber; said electrodes being symmetrically disposed with respect to the axis thereof; means for electrically connecting alternate ones of said electrodes together to form two electrode pairs; said electrode pairs having direct current potentials of an opposite polarity applied thereto; wherein the improvement comprises: a source of high frequency sinusoidal voltage of variable amplitude V; means for producing a d.c. voltage proportional to the amplitude V of the high frequency voltage; variable resistor means forming a voltage divider for said d.c. voltage and for providing a second d.c. voltage of a value Alpha MV, where Alpha M denotes a critical U/V ratio at the limit of stable spectrometer response; means for biasing said second d.c. voltage of magnitude Alpha MV with a third d.c. voltage of magnitude -K, where K denotes a constant which determines the peak width Delta m of a mass spectrum; and means for applying the high frequency voltage and the resultant d.c. voltage of magnitude Alpha MV-K to said electrodes of said quadrupole structure.
 3. A variable resolution system for a quadrupole mass spectrometer comprising an ionization chamber, an ion detector and a quadrupole structure having four electrodes extending between said detector and chamber; said electrodes being symmetrically disposed with respect to the axis thereof; means for electrically connecting alternate ones of said electrodes together to form two electrode pairs; said electrode pairs having direct current potentials of an opposite polarity applied thereto; wherein the improvement comprises: a source of high frequency sinusoidal voltage of variable amplitude V for application to said electrodes of said quadrupole structure; means for generating a d.c. voltage U for connection to said electrodes which remains at a constant value of H for values of V, wherein O<V < or = K/ Alpha M-H and which varies in accordance with the equation U Alpha MV-K for a value of V in excess of K/ Alpha M-H, where Alpha M denotes a critical U/V ratio at the limit of stable sPectrometer response and K denotes a constant determining the peak width Delta m of a mass spectrum determined by said spectrometer; means for varying the parameter K to vary peak width; and means for varying the parameter H to establish said parameter H at a value smaller than the peak value of the stable spectral response for atomic hydrogen.
 4. A variable resolution system for a quadrupole mass spectrometer comprising an ionization chamber, an ion detector and a quadrupole structure having four electrodes extending between said detector and chamber; said electrodes being symmetrically disposed with respect to the axis thereof; means for electrically connecting alternate ones of said electrodes together to form two electrode pairs; said electrode pairs having direct current potentials of an opposite polarity applied thereto; wherein the improvement comprises: a source of high frequency sinusoidal voltage of the variable amplitude V; means for generating a d.c. voltage proportional to the amplitude V of said high frequency voltage; voltage divider means for deriving, by voltage division of said d.c. voltage, a second d.c. voltage of a value Alpha MV, where Alpha M denotes a critical U/V ratio at the limit of stable spectrometer response, means for biasing said second d.c. voltage with a third d.c. voltage of a value K, where K denotes a parameter that determines the peak width Delta m of a spectrum; means for rendering said biased d.c. voltage Alpha MV - K equal to a null output for a value of V between zero and K/ Alpha M; means for comparing the output from said rendering means with a constant voltage H and choosing the greater of the two as its output U, the value of H being chosen less than the peak value of the stable spectral response for atomic hydrogen and means for applying said output U and said high frequency voltage to the electrodes of the quadrupole structure.
 5. A variable resolution system according to claim 4, further including means for varying the magnitude of the voltage H. 